The present invention relates to a method of fabricating a unitary coaxial pipe assembly element, in particular for undersea pipes conveying hot or cold fluid, preferably undersea pipes for use at great depths.
In most industrial fields, it is desirable to obtain insulating systems of high performance in order to maintain fluids conveyed in pipework at a constant temperature, so that transfers between pieces of equipment can be achieved over long distances, e.g. reaching several hundreds of meters or even several kilometers. Such distances are commonplace in industries such as oil refineries, liquefied natural gas installations (−165° C.), and undersea oil fields of the kind extending over several tens of kilometers. Such oil fields are being developed in ever-increasing depths, which can exceed 3000 meters (m).
The present invention relates in particular to coaxial pipe elements for use in fabricating undersea pipes that are installed over oil fields at very great depths, in particular bottom-to-surface connection pipes that are suspended between the bottom of the sea and a surface vessel anchored over said oil field.
Such coaxial pipes are referred to by the abbreviation PiP (for pipe-in-pipe), and they have both an inner pipe for conveying the fluid and an outer pipe placed coaxially around the inner pipe, also referred to as the “outer shell”, that comes into contact with the surrounding medium, i.e. sea water. The annular space between the two pipes can be filled with an insulating material or it can be evacuated of any gas.
Such systems have been developed to achieve a high level of thermal performance, and specific versions have been developed that are better adapted for use at great depths, i.e. that are capable of withstanding the pressure at the sea bottom. Given that pressure under water is substantially equal to 0.1 megapascals (MPa), i.e. about 1 bar, for every 10 m of depth, the pressure that the pipe needs to be capable of withstanding is then about 10 MPa, i.e. about 100 bar at a depth of 1000 m, and about 30 MPa, i.e. about 300 bar at a depth of about 3000 m.
Such coaxial pipe assemblies are made by end-to-end assembly of unit lengths referred to below as “coaxial pipe elements” or as “coaxial pipe strings”, of length that generally lies in the range 10 m to 100 m, and more particularly that is equal to about 12 m, 24 m, or 48 m, each.
In the context of installing undersea pipes at great depths, these unit length elements are fabricated on land. They are then transported to sea on a laying vessel. While being laid, the unitary coaxial pipe assembly elements are connected to one another on board the vessel progressively while they are being laid at sea. It is therefore important for the making of such connections to be suitable for incorporation in the method of mounting and assembling the pipe and laying it on the sea bottom with as little delay as possible, and for connections to be made quickly and easily.
For this purpose, use is made of junction pieces, i.e. steel connection forgings, that are assembled to the ends of said coaxial pipe assembly elements that are to be assembled together. The junction forging at the downstream end of a first as-yet unassembled coaxial pipe assembly element is connected to the junction forging at the free upstream end of a second coaxial pipe assembly element that has already been assembled at its downstream end.
These junction forgings also serve to reinforce the strength of pipes that are subjected to high levels of bending during laying, in particular in the connection zones between two said successive unit lengths, and more particularly for bottom-to-top connections or “rises”, they serve to give them very great resistance to fatigue throughout the lifetime of such installations.
More particularly, the present invention relates to said junction forgings comprising two branches of revolution, comprising an outer branch and an inner branch that together form a fork defining said annular space, with the cylindrical free ends of the fork being assembled directly to the cylindrical ends of the outer and inner pipes, respectively.
Coaxial pipes and junction forgings of that type are described in particular in FR 2 873 427.
A fundamental operation for ensuring the mechanical reliability of PiP pipes, lies in the welds between the junction forgings and said coaxial pipes. In particular, welders must be capable of monitoring the welding that is being performed, and also after it has been performed, in particular with the help of weld inspection devices using ultrasound probes, which devices can be operated by an operator either manually or using a robot, and in any event the probe must be moved against and close to the weld, both axially in forward and backward translation over the weld zone and circumferentially around the entire periphery of the pipe in said weld zone.
That is why it is desirable to be able to perform welding between the junction forgings and the coaxial pipes from outside the pipes in question so as to make the welding easier to monitor. However, with junction forgings of the type comprising two branches of revolution having cylindrical ends that are assembled to the respective cylindrical ends of the inner and outer pipes, such welding from the outside is not possible for welding together the inner branch of the junction forging and the end of the inner pipe, at least for one of the two junction forgings, so welding must then be performed from inside said inner pipe, as explained below. Unfortunately, this welding operation from inside the inner pipe is difficult and complex to perform, and it is likewise difficult and complex subsequently to inspect the weld. It will be understood that when welding from the inside, an operator has great difficulty in positioning accurately either the welding torch or the inspection device.
As mentioned above, welding zones are particularly sensitive to the phenomenon of fatigue, both during laying and during the lifetime of the pipe, which is why it is important to be able to inspect reliability with great care.
In order to be able, from the outside of the inner pipe, to weld the inner branch of the junction forging to the end of the inner pipe, and in order to be able to do so at both ends of a unitary coaxial pipe element, one solution is to make junction forgings as a plurality of parts and/or to interpose fittings, in particular tubular half-sleeves that form two tubular half-shells that are interposed between the end of the outer branch of a junction forging and the corresponding end of the outer pipe, as shown in FIG. 1C. These half-shells are welded between the ends of the outer branch of the junction forging and of the outer pipe, and they are put into place after the inner pipe has been welded from the outside between the end of the inner branch of the junction forging and the end of the inner pipe.
However, those junction forgings that are complex and/or associated with additional elements of the shell type affect the mechanical reliability of the junction forging itself, and thus of the junction between the junction part and the pipe. One of the reasons is because it is necessary to perform welding in the longitudinal direction of the pipe in order to weld together the two tubular half-sleeves, even though this type of longitudinal welding is less reliable than is circular welding, and above all involves crossed welding between the circular welds interconnecting the tubular sleeves to the junction forgings or to the coaxial pipe ends and the longitudinal welds interconnecting the two additional tubular half-sleeves, said crossed welds constituting additional points of weakness.
Systems are also known in which the junction piece is made from two elements that are screwed and adhesively bonded together, but that type of junction piece also suffers from poor mechanical reliability.
FR 2 751 721 discloses a method of making the ends of a PiP associated with a method of reinforcing the connection zone between two unit lengths of PiP by means of a sliding sleeve presenting little clearance relative to the outer pipe, said sliding sleeve being secured to said outer pipe by adhesive. That disposition serves to increase the second moment of area of the cross-section locally so as to limit stresses in the coupling zone between two unit lengths of PiP, but it requires several mechanical parts to be fabricated that are complicated to mount and that require connection to be performed in a manner that is relatively difficult. In addition, the adhesive proposed remains subject to creep and it deteriorates during the thermal cycling to which pipes are subjected during a lifetime of 20 years to 30 years. Finally, that type of adhesive cannot be considered as being reliable for bottom-to-surface connections since the dynamic effects of swell and of currents on the pipes suspended between the floating support and the sea bottom rapidly degrade the adhesive plane, giving rise quickly to excessive fatigue in the PiP connection zone.
Thus, the problem posed is that of making a connection to a unit length of a PiP type coaxial pipe assembly that is improved in terms of making it easier to install the connection means and to perform the connection operations, in particular in terms of welding; and in which the connection zones, in particular the weld zones between junction forgings and unit pipe lengths can be provided in such a manner that the stresses generated during laying are minimized and the fatigue behavior in bottom-to-surface connections is greatly improved.
More particularly, a problem lying behind the invention is that of providing an improved method of fabricating a coaxial pipe element that includes, at each end, a junction forging constituted by a single forging having two branches of revolution, an outer branch and an inner branch, that are assembled directly to the ends of the outer and inner pipes, respectively.